The present invention relates to detecting the presence of and identifying a contaminant belonging to a family of known contaminants, the contaminant being on the surface of a structure, and the invention makes use of the difference in acoustic impedance between air and contaminants such as ice.
A major application of the invention lies in aviation, where, during cold periods, it is important to discover prior to aircraft take off whether the surfaces of its lift structures are clean, and, if they are polluted, to determine the nature of the contaminant (ice, mixture of solid and liquid phases, liquid, etc.).
Ultrasonic devices are already known for detecting ice and optionally also for measuring the thickness of a layer of ice. They do not make it possible to identify a contaminant of some other kind that may happen to the present. Unfortunately, prior to aircraft take off, it is important to determine whether the wings are covered by ice which severely and lastingly reduces lift, or whether they are covered of melting snow, of an antifrost or a defrosting liquid, or of a mixture of water and defrosting or antifrost liquid that will be blown away on acceleration.
There is also known (U.S. Pat. No. 2,966,058) a method for measuring dynamic properties of material, wherein the amplitude and phase of acoustic energy shear waves which have traversed a block are compared with the amplitude and phase of waves reflected by a surface of the block which carries a sample of the material.